Field of the Invention
The present invention relates to a sensor device including a cover member which covers an opening provided to a casing and a fixing cover which fixes the cover member to the casing.
Description of the Background Art
A sensor device may need to have a casing with an external surface partially composed of a member different from the body of the casing, and in that case, the member is assembled to externally cover an opening provided to the body of the casing for example. In that case, in order to externally seal the casing's internal space to ensure resistance to the environment, an assembly structure is generally adopted in which a gasket is interposed between the above member and the casing's body and in that condition, a fixing cover is used to fix the above described member to the body.
For example, Japanese Patent Laying-Open No 2007-221007 discloses a case in which the above described assembly structure is adopted to assemble to a casing's body a transmissive plate which covers a light projecting and receiving opening of a photoelectric sensor serving as a sensor device. In the photoelectric sensor disclosed in the publication, an assembly structure is adopted which fixes a fixing cover to the casing's body by so-called snap fit composed of an engagement tab and an engagement hole.
However, the assembly structure as disclosed in the above publication has a configuration such that a tolerance of the engagement tab and hole per se, a tolerance of the casing's body and fixing cover provided with the engagement tab and hole, and furthermore, a tolerance of the gasket, etc., i.e., various parts' tolerances for implementing the assembly structure would be accumulated.
Accordingly, it would be significantly affected by variation in dimension of these parts and it is not easy to ensure high resistance to an environment, and in a relatively severe environment where a cutting oil or a similar oil, a significantly corrosive agent etc. are used in large amounts, stable sealing performance cannot be ensured over a long period of time.
Furthermore, when the above described tolerance accumulation is considered, then, in order to absorb various parts' dimensional variation, it would be necessary to increase the gasket in size to ensure a large deformation margin, which would obstruct miniaturization of the sensor device.
Furthermore, in order to firmly fix the fixing cover to the casing's body by using snap fit against the gasket's resilience, the fixing cover is required to have a sufficiently long distance from a portion which covers a member to be fixed to a portion provided with the engagement tab or hole, and the sensor device's external design may be impaired.